in all honesty here lies a rather big one
have Leon Trotsky be the leader of the soviet union. This is the big one for a few reasons
reason 1 fascism and importantly Hitler comes to power. he still hates communists and Jews and guess what Trotsky is both
reason 2 Trotsky still believes in the world revolution and sees Hitler as the biggest opponent and depending on how things go he probably attacks first
reason 3 world revolution believing Trotsky probably funds communist revolutionaries all over the world and this does not endear him to anyone and with his attack on Germany the rest of the world supports Germany making communism the enemy of ww2 at least
It might work but Trotsky coming to power is actually surprisingly hard.
IMO the best POD might be no miracle on the Vistula resulting in the CCCP integrating basically all of Poland from 1920. Lenin pushes for further advance into Germany but Trotsky persuades him that, yeah, there's no way the Red Army can do that. Instead he persuades Lenin to cede the Danzig corridor back to the Germans and the KPD gains a huge base of support from what IOTL was the "left" of the Nazi party--the Communists have shown that they can nullify the humiliation of Germany after all by playing nice with the CCCP. Ultimately the KPD is able to take power.
It might be possible for Italy to go Communist rather than Fascist during the Bienno Rosso, otherwise, Mussolini might try to tack back to his Socialist roots to ingratiate Italy with the Comintern--though whether that gets anywhere is another matter, and probably depends on who succeeds Lenin (Stalin would allow it, as would possibly the rest of the right Bolsheviks, most of the Left wouldn't).
France, conversely, would see the Commies re-strengthening Germany and would crack down hard on its own Communists.
In Spain, with both Germany and the CCCP as well as
possibly Italy supporting the Republic, the coup of 1936 fails and the Leftists crack down hard on the Right. The Spanish Republic moves towards a Comintern affiliation.
From here, you can get a WWII between the Allies and the Comintern, with factions looking something like the following IMO:
Allies:
--France
--Britain + Commonwealth
--America may join in if USW happens, but see below for why I find this unlikely.
Comintern:
--CCCP
--Spanish Republic
--German People's Reich
--Possibly Italy
Neutral:
--Italy
--possibly Japan
--possibly USA
Toss-up:
--Japan
--China
As WWII starts, I imagine that the Soviets will argue that the better way to cripple the UK without involving the USA is going to be an invasion of India and the Middle East, cutting the Suez Canal and the British oil supply. If they can restrain the Germans from launching USW, they likely will not draw the USA in by this expedient.
Things get interesting when you start to look at Asia. Since both Germany and the CCCP IOTL heavily backed the KMT and Chinese Republic, I find it likely that the USA and Britain look to Japan as a counterweight to the Comintern in East Asia. Thus, an embargo is highly unlikely, indeed, both countries will probably funnel resources towards the Empire. Depending on what happens at ATL Khalkin Gol, Japan may declare war on the USSR, but I believe that this is unlikely purely because there aren't that many resources to justify a full on invasion of Siberia.
The other Asia scenario would be butterflies resulting in an earlier resumption of the Chinese Civil War, and the Comintern going all-in for the CCP. From here, the KMT/RoC are pro-Allies, and by domino effect the Japanese pro-Comintern. Here, it is much more likely that Japan attacks Britain and the USA as OTL and from there the USA becomes involved against the Comintern.
In the first scenario, I believe that the Comintern will win 7 times in 10. The combined Red Army and Volkswehr will be able to destroy the French with ease, and from there it is likelier than not that the Communists can push into the Middle East or India and sever British oil supplies. They certainly won't take Britain, but they will dismantle the Empire and set up pro-Comintern regimes in the Middle East and India. However, there is a chance that the highly defensible terrain in Iran will slow down Soviet advances.
In the second, it all comes down to when the USA joins in and how much energy the Soviets use to prop up Japan. The USA IMO will not want to stomach the long and bloody war that it would take to push all the way into Russia, particularly if the Soviets don't Pearl Harbor or similar. Given that the Reds will control most of the Continent, canned sunshine won't be an easy option to hit the Motherland (there's also the fact that most Russian cities are spread out enough that nukes will have minimal impact). Ultimately, in this scenario, I believe that either a stalemate or a signed peace in which the Allies make some gains--possibly carving off Soviet Central Asia as a British Mandate but not much else--is the likeliest outcome, but it could go many ways.